If your parents hurt you...call the cops. If you know for sure your stepfather is going to go ballistic, try to have a one on one conversation with your mother first. Also try to have someone else in attendance when you break the news, a neutral person or someone they have great respect for would be best as then they will hopefully not react violently
You may want to contact a local planned parenthood organization. They can help you weigh your choices. They might also be able to provide information on services which are available to you for a place to stay until the baby is born.